Inheritance Flashcards
What is a gene?
-A sequence of DNA bases (arranged as triplets) that codes for a particular polypeptide (each triplet codes for one specific polypeptide chain)
What are two characteristics that genes (alleles) have?
-can separate and combine (during sexual reproduction/fertilisation)
- can mutate (change DNA base sequence)
What are homologous pairs of chromosomes?
A homologous pair of chromosomes consists of two chromosomes, one from each parent, that are similar in size, shape, and genetic content, carrying genes for the same traits.
What are alleles?
alternative forms of genes occupying the same gene position (or locus) on homologous chromosomes
what are the three different (theoretical) allele combinations for any single gene?
Heterozygous, homozygous, dominant, homozygous recessive
In homologous chromosomes, where is each chromosome originated from?
- one originated from the egg
- one originated from the sperm
What do alleles of the same gene do?
occupy the same locus
what is heterozygous?
Having different alleles for a given gene (where a dominant allele and a recessive allele are present together). Each allele is carried on a different chromosome within a pair of homologous chromosomes. E.g. Bb
What is homozygous dominant?
Having the same two dominant alleles present for a given gene. Each allele is carried on a different chromosome within a pair of homologous chromosomes. E.g. BB
What is homozygous recessive?
Having the same two recessive alleles present for a given gene. Each allele is carried on a different chromosome within a pair of homologous chromosomes. E.g. bb
What is the gene pool?
All the alleles in a population at any one time
How is a dominant allele expressed?
always expressed in the phenotype (physical appearance of the organism)
How is a recessive allele expressed?
only expressed in the phenotype (physical appearance of the organism) when no dominant allele is present
What is definition of genotype?
-The genetic make up of an individual; all of the alleles that they possess
-These alleles dictate their observable characteristics (phenotype)
what is the definition of phenotype?
The observable characteristics of an organism determined by the genotype
What is a particular characteristic controlled by and what does this mean? (genotype and phenotype)
a particular characteristic is controlled by a single gene therefore if an organism is homozygous dominant or heterozygous for a particular characteristic, the dominant allele will always be expressed in the phenotype
What happens when a homozygous dominant flower with purple petals is crossed with a homozygous recessive flower with white petals?
each gamete from each flower provides one allele to the offspring
What does mono hybrid mean?
one gene
what is a test cross?
used to determine an unknown genotype when the dominant phenotype is present (cross unknown phenotypically dominant plant with homozygous recessive plant and look at offspring: Are they dominant or a one-to-one ratio of homozygous, dominant and homozygous recessive phenotypes?)
What is monohybrid inheritance?
The inheritance of a single gene such as that controlling plant height or seed colour
What did mendel investigate?
Inheritance in plants of the garden pea
Why were the plants a useful choice for inheritance experiments?
•Easy to grow / large numbers of offspring quickly
•Can self or cross fertilise
•Produce flower and fruit in the same year (see phenotypes quickly)
What did Mendel do and what did he find?
-chose pairs of conntrasting characteristics such as tall or dwarf plants, Round or wrinkled seeds and yellow and green seeds
-He was fortunate that his choice of characteristics were controlled by single gene, and were easy to tell apart
What are three characteristics of a gene?
•Has a sequence of codons that specifies the sequence of amino acids in a polypeptide chain
•Genes can separate and combine (during sexual reproduction)
•Genes can mutate (change DNA sequence
When crossing plants how would mendel ensure that the offspring produced were only from the parents he chose?
•Pollen transferred by hand - receiving flower could be in a bag to prevent further cross-pollination
•Remove anthers before they mature so no self-pollination
What is mendel’s first law of inheritance?
law of segregation- the characteristics of an organism are determined by factors (alleles) which occur in pairs.
Only one allele of a pair is present in each gamete
What is continuous variation controlled by?
a number of genes i.e. a collection or number of genes might contribute to how tall a person becomes not just a single gene
What is the characteristic in continuous variation?
-Characteristic often has a range of values (Usually the bell shaped normal distribution curve)
-Controlled by a number of genes (polygenic), e.g. if height is influenced by two or more genes, then they have the potential to grow tall
-
What have an effect on a characteristic in continuous variation?
Environmental factors
What is discontinuous variation controlled by?
Discontinuous variation is usually controlled by a single gene, there are no intermediate types for example ABO blood types
In discontinuous variation, what does the characteristic have?
Characteristic has distinct categories, controlled by a single gene and environmental factors have no effect
What can influence genetic variation other than genes?
Environmental factors can influence expression of genes, for example they may turn genes on or off = called epigenetics (gene expression)
What is the ‘backcross’ or test cross?
a method used in genetics to determine whether a particular dominant characteristic observed in an organism is controlled by one dominant (heterozygous) or two (homozygous dominant) alleles. The unknown is always crossed with a recessive phenotype.
What is pure breeding?
Two organisms of the same phenotype, that when bred together, produce offspring with the same phenotype (homozygous)
What is co-dominance?
condition in which both alleles for a gene are dominant and expressed in the phenotype when present in a heterozygote (both alleles are written using a capital letter)
In co-dominance how does the heterozygous individual express both alleles?
In co-dominance a heterozygous individual expresses both alleles simultaneously without any blending. (Animals with stripes/spots tend to have codominant alleles)
What is incomplete co-dominance?
one allele is not completely dominant over the other. The heterozygous condition is somewhere inbetween the two homozygous phenotypes
How would you explain why something is an example of incomplete dominance?
heterozygous individuals have a blended phenotype where (red) and (white) are mixed to make (pink) flowers
What is dihybrid inheritance?
-It involves the inheritance of two unlinked genes (genes found on different chromosomes). -Independent assortment of these genes produces recombinants (different allele combination in the gametes)
What is Mendel’s second law (law of independent assortment) ?
Either allele from a pair of alleles may combine randomly with either allele from a second pair of alleles
What are expected offspring ratio’s?
(theoretical) done before that actual cross itself
What are observed ratios?
Actual offspring ratios (chi squared)
What do genetic crosses give and what happens when the cross is actually carried out?
-give expected offspring ratios. -however when the cross is actually carried out the ratio of offspring produced could be different
What are the chi-squared key terms?
Expected (E)
Observed (O)
What is expected?
Theoretical phenotype ratios produced through genetic crosses (making a punnett square before actual cross has been done)
What is observed?
Actual phenotype ratios produced once the genetic cross has been completed
What can a Chi square test be used to do?
to work out if the expected ratio is significantly different to the observed ratio
What is needed at the start of a chi-squared test?
Null hypothesis
What is a null hypothesis?
there is no significant difference between the observed and the expected phenotype ratios and any difference is due to chance
What do we do if there is no significant difference between the observed and expected ratios?
We accept our null hypothesis- any difference is down to chance
What do we do if there is a significant difference between the observed and expected?
Then we reject the null hypothesis – other factors must be having an influence
When can chi-squared only be used?
If the data is categorical
What is the chi square equation?
chi-sqaured= (o-e)2/e
(must be done for each phenotype, and then results should be added together)
What is P?
Probability that the difference between zero and expected ratios are all due to chance
What is the value for degrees of freedom?
One less than the number of categories (i.e phenotypes we’ve used)
In biology, what do we always compare our value to?
The value of 5% significance level (0.05)= Critical significance value
What happens if the chi-squared value is less than the value in the table?
We accept the null hypothesis therefore any difference is down to chance
What do we do if the Chi, squared value is greater than the value in the table?
We reject the null hypothesis therefore, other factors are influencing.
Where are genes for particular traits found?
in same location on chromosomes (locus) in different individuals of the same species
What would be the conclusion for chi squared?
chi-squared value was … and the critical value from the table was .. (for a P value of 0.05)
•Our chi squared value is bigger/smaller and so P <0.05/ P >0.04
•There is/isn’t a significant difference between observed + expected ratios
•We reject/ accept the null hypothesis
•Some other factor must be having an effect / results due to chance
What are the two types of chromosomes?
-autosomes
-sex chromosomes
What are autosomes?
all the chromosomes inside a nucleus excluding the sex chromosomes
What are sec chromosomes?
chromosome concerned in determining the sex of an organism typically one of two kinds.
In humans, they are similar in one sex (females) and dissimilar in other sex (males)
What type of chromosomes are the 23 pairs of chromosomes in humans?
22=autosomes
23=sex chromomes
what are described as sex-linked (or X-linked)?
Alleles that are carried on the X chromosome
why does the male chromosome carry fewer genes?
it is smaller than the X so carries less genes
For most genes carried on X chromosome in the male what is the only one of?
one allele which must be expressed therefore in the male any recessive genes on the X chromosome will be expressed in the phenotype
If a male with a sex linked disease had a son, what are the chances of him having the disease?
Sons (male) only inherit the recessive allele on their X chromosome from their mother as they inherit the Y chromosome from their father. Only one recessive allele is needed in the son for it to show in the phenotype
What would happen if a male with a sex linked disease had a daughter?
•All daughters (female) can inherit the recessive allele from their mother or father but require 2 recessive alleles for it to show in the phenotyp
What are examples of sex linked diseases?
•Colour blindness = affects 1/20 males but only 1/400 females
•Muscular dystrophy
•Haemophilia = an example of a sex-linked condition in humans
Why my women were a carrier mother and unaffected father one genetic counselling before having children?
to find out if they have inherited the recessive allele as this would make them carriers, if they passed the recessive allele onto their son they would have to disorder. They may decide to screen the embryo when they become pregnant.
what is haemophilia?
-an example of a sex-linked condition in humans
-the individual can’t produce enough of one particular blood clotting protein
what could be a symptom of haemophilia?
slow, persistant bleeding
What is Duchenne muscular dystrophy caused by?
-a sex linked recessive allele of the dystrophin gene
-genes codes for the proteins dystrophin, which stabilises the cell membranes of the muscle fibres
What would the symptoms for individuals with duchenne muscular dystrophy be?
Loss of muscle mass and muscle weakness
What does a chromosome contain?
contains a linear sequence of genes which are all linked together. Genes on the same chromosome are therefore usually inherited together
what does it mean if genes are on different chromosomes?
they are unlinked
when does Mendel’s ratios only apply? (9:3:3:1)
When genes are on different chromosomes (not linked)
Why do mendels ratios not apply to genes in same chromosomes?
genes on the same chromosome are linked and usually inherited together
when does crossing over disrupt linkage?
5-10% of the time
what can crossing over affect?
the linkage of genes and leads to incomplete linkage
What stage of meiosis does crossing occur?
prophase 1
when are genes more likely to be affected by crossing over?
if they are further apart on a chromosome
What is incomplete linkage?
the further apart two genes are on a chromosome, the more chance there is of crossing over taking place
In which of the two examples below are genes 1 and 2 more likely to be separated by crossing over? Explain your answer
Second example as the two of the genes are further apart on the chromosome so crossing over in between them is more likely to take place
What is a linkage?
-genes are inherited together. -Crossing over is very unlikely to separate them as the genes are too close together on the same chromosome.
what is does incomplete linkage?
-genes are on the same chromosome, but are far apart. -Genes could be separated due to crossing over and not be inherited together but this is rare (only 5-10% of the time).
In another cross between two individuals with the genotype Dd Ee, where the genes D and E are on the same chromosome, the offspring showed four different types of phenotype.
The phenotype of some of the offspring were far more common than expected and some phenotypes were very rare. Explain these observations.
Incomplete linkage;
Genes (further/ far) apat on same chromosome;
{Crossing over/ chiasmata) can occur;
→ Four types of gametes produced but not in equal numbers);
→ Small numbers of recombinants / large numbers parental
types;
→ Recombinants equal in numbers / parental equal in numbers;
second part of q-
If the genes were on same chromosome they are linked and we might expect two phenotypes in a rato 3:1, but there must be incomplete linkage here because there are
four phenotypes = two alleles on same chromosome separated by crossing over
what is the definition of a mutation?
An unpredictable change in the genetic material of an organism which changs the base sequence coding for a polypeptide chain
What are the two types of mutations?
•Gene mutations - affect individual / single genes
•Chromosome mutations - cause changes in the structure or number of whole chromosome
What is the importance of mutations?
Mutation rates are low but in organisms with short life cycles and frequent cell division, they are more frequent.
•Mutations most often occur during crossing over in Prophase I, and non-disjunction during anaphase I and anaphase II.
What scientists found that mutation rates do?
increase if organisms are exposed to mutagens
what are mutagens?
factors in the environment which increase the frequency of mutation in an organism
what are examples of mutagens?
•Radiation e.g. UV radiation and gamma radiation
•Chemicals e.g. heavy metal, polycyclic hydrocarbons
Why is it incorrect to say that mutagens cause mutations?
Mutagens do not always cause mutations directly but rather increased exposure to mutagens increases the rate at which mutations occur
•For example, a mutagen may interfere with DNA replication enzymes leading to inaccurate DNA replication which may lead to mutation
What are carcinogens?
mutagens that increase the rate of cancer
What do proto-oncogenes and tumour-suppressor genes do?
regulate cell division within humans
if these genes become mutated, they form oncongenes
what are oncogenes involved in?
uncontrollable cell division and can form cancer.
What do proto-oncogenes do?
Proto-oncogens normally stimulate cell division in a healthy way
mutations in prote-oncogenes can result in an “oncogene” The oncogene now causes the cells to divide too rapidly
what do tumour suppressor genes do?
-help regulate division by slowing down cell division in healthy cells. -Mutations in a tumour suppressor gene can result in cells dividing too rapidly.
What are some harmful chemicals that tobacco smoke contains that affect human health?
-carbon monoxide
-ammonia
-aresnic
What does tar do?
-Target the DNA in the cells of the alveoli
-If mutation occurs in a proto-oncogene or a tumour suppressor lung cancer may develop
what do gene mutations affect?
Single bases within a gene
What could a change in a single base of a gene result in?
-change in DNA base sequence resulting in a change in triplets/codes for amino acids
-Different amino acid sequence in translation
-Bonds form in different places and result in a different shaped protein (may not function)
An enzyme is required for melanin (pigment) to be produced. Q3. Explain how a mutation in the gene coding for this enzyme results in a loss of pigmentation in albinos.
Changes to the DNA base sequence results in a change in the sequence of amino acids during translation
•A mutation in the gene coding for the enzyme could result in the active sight of the enzyme changing shape.
•The enzyme cannot function and no melanin (pigment) is produced.
where is an example of a substitute gene mutation found?m and what does this mean
in the haemoglobin gene- The replacement of only one base pair in the DNA results in the change of just one amino acid in two of the polypeptide chains making up the haemoglobin molecule. The abnormal haemoglobin causes the red blood cells to become sickle shaped, affecting the ability of the haemoglobin to bind oxygen. This results in anaemia and may be fatal.
what are the two possible alleles for sc)
HbA (normal)
Hbs (sickle cell)
what is the mutant allele in sickle cell?
codominant
Sickle cell trait also confers some resistance to the malaria parasite, why do you think this is the case?
The malarial parasites (Plasmodium) reproduce inside red blood cells but cannot reproduce inside sickle shaped cells = so having sickle cell trait could be a selective advantage (don’t get malaria)
what are chromosome mutations?
Mutations causing changes in structure or number of whole chromosome
When are chromosome mutations likely to occur?
during meiosis, when the process can go wrong as the paired chromosomes (bivalents) line up on the equator at metaphase I and are pulled apart during anaphase I (due to contraction of the mitotic spindle fibres)
what are the types of chromosome mutations?
Changes in chromosome structure
- changes in chromosome structure
Errors occur when chromosomes re-join with corresponding exchanged sections of DNA during crossing over at prophase I
what are the types of chromosome mutations? (2)
- Changes in sets of chromosomes (polyploidy)
If a gamete receives two sets of chromosomes during a failed meiotic division, the gametes will be diploid (2n) rather than haploid (n)
What would this then result in a fertilisation occurred?
-When this diploid gamete is fertilised by a haploid male gamete, the zygote may be triploid (3n)
•or tetraploid (4n) if two diploid gametes fused
Explain how a trisomy, such as trisomy 16 in mice, can occur in a zygote.
In the primary oocyte, homologous chromosome pairs (making a bivalent together) fail to separate - called non-disjunction
•Occurs at anaphase I (during oogenesis in the ovary)
•So the secondary oocyte has two copies of chromosome 16
•A third chromosome 16 added from the sperm nucleus at fertilisation
what is epigenetics?
the control of gene expression by modifying DNA or histone proteins, but not affecting the DNA nucleotide sequence.
what does evidence show and what are these changes?
that the environment can alter the expression of genes by affecting how they are transcribed. These changes are epigenetic – they affect the genes but not their nucleotide sequences.
When can DNA be modified and what does this do?
-DNA can be modified post-replication.
-This changes the ability for a gene to be transcribed in protein synthesis.
What are the two ways that DNA can be modified post replication?
-Addition of methyl group to nitrogenous bases.
-Modification of histone proteins
what does addition of methyl group to nitrogenous bases do?
This prevents these bases being recognised and therefore reduces the ability of the whole gene to be expressed.
What happens when methyl groups are added to DNA bases?
the RNA polymerase can’t bind and so transcription does not occur = no MRNA = no polypeptide chain = gene SWITCHED OFF
What are his proteins used to do? (modification of histone proteins)
.
These proteins are used to organise the DNA in a chromosome.
what happens if a DNA becomes more tightly/loosely coiled around histone proteins?
this can prevent gene expression.
If coiled more loosely it can increase gene expression
what are the types of chromosome mutations? (3)
Non-disjunction is a process in which faulty cell division results in one daughter cell getting two copies of a chromosome, whilst one daughter cell gets none (of that particular chromosome)
What causes different expression of the same gene within one individual organism?
Different epigenetic modifications can occur in cells of the same tissue and in different tissues of the same organism.
What is a locus?
The particular region that the gene occupies on a chromosome
What does F1?
The first filial generation
What is F2?
The second filial generation
What is affected in down syndrome?
chromosome number 21 is affected. During meiosis in the ovary, an oocyte with no copy of chromosome 21 is formed which dies. An oocyte with two copies of chromosome 21 survives
what happens if the oocyte that survives is fertilised?
zygote will have 3 copies of chromosome 21, i.e 24 chromosomes.
The resulting individual will have Down’s syndrome with open facial features, too large a tongue, heart defects and learning difficulties.
What is changes is chromosome number (disjunction)
Nondisjunction is a process in which faulty cell division results in one daughter cell getting two copies of a chromosome, whilst one daughter cell gets none.
